While conventional brush-commutated DC motors may have advantageous characteristics, including convenience of changing operational speeds, there may be disadvantages such as brush wear, electrical loss, noise and radio frequency interference caused by sparking between the brushes and the segmented commutator, which may limit the applicability of such brush-commutated DC motors in some fields such as the furnace blower control field. Electronically commutated motors, such as brushless DC motors and permanent magnet motors with electronic commutation, have now been developed and generally are believed to have the above discussed advantageous characteristics of the brush-commutated DC motors without many of the disadvantages thereof while also having other important advantages. Such electronically commutated motors are disclosed in the David M. Erdman U.S. Pat. Nos. 4,015,182 and 4,459,519, for instance. These electronically commutated motors are advantageously employed, for instance, in various air handling applications such as air conditioning for cooling and warming.
In a conventional furnace, considerable heat energy is wasted when it is exhausted to the atmosphere. This makes the overall efficiency of the system poor considering the BTU content of the fuel. Efficiency and fuel economy can be greatly improved by extracting the heat from the furnace exhaust. Natural convection of the hot exhaust causes it to rise and vent to the atmosphere. In order to improve efficiency and economy, the heat is extracted from the exhaust by a heat exchanger in which case additional pressure is needed to force the cooled exhaust to vent to the atmosphere. This is accomplished by inducing a draft.
In a draft inducer control system, such as used in high efficiency furnaces, a variable resistance can be used to vary the speed of a brush-type fan motor to induce drafts, but this would further reduce the energy efficiency of the system. While there are some losses engendered by electronic switching of an electronically commutated motor, these are negligible compared to brush losses and rheostat losses in prior art variable speed draft inducer systems.
Further improvements in control systems, electronically commutated motor systems, draft inducer apparatus and methods of control and operation can beneficially contribute to more widespread use of such motors in various applications including fan control for inducing drafts in high efficiency furnaces. For example, sudden changes in resistance to draft and line voltage variations can lead to reduced drafts or excessive drafts which adversely affect furnace efficiency and product potentially dangerous backdrafts or over-drafts. Improvements which achieve increased torque and speed control would be desirable. Economy of manufacture would also be enhanced by circuit improvements if they can be made with little extra cost as part of improved integrated circuit chips. Greater versatility of response to various control signal conditions and improved fail-safe features would also be desirable.